The proposed seeks funds to continue and expand two twin studies of infants and toddlers. The fine-grained, longitudinal Genetics of Emotional Ontogeny (GEO) includes multi modal, comprehensive assessment of emotion and temperament as well as selective assessment of cognition, motor development, physiology, social interaction, and the home environment from birth to age 3 years. The project incorporates an unusually broad set of methods, including lab-based elicitation of behavior, home observation, testing by examiner, telephone interviews, diaries, narrative constructions, questionnaires, hospital records, cortisol assays, and central (EEG) and periphal (cardiac) psychophysiology. The second twin study uses the Wisconsin Twin Panel, with a base population of all young twins born in the state of Wisconsin; it pursues goals that overlap highly with GEO, but with a sample of over 2000 pairs of twins and more economical assessment. Additionally, the large sample size of WTP allows study gender-related differences and similarities in emotional development and identification of subgroups of children with extreme temperaments for further study. The chief issues addressed are the nature, sources, and functional consequences of emotional individuality. The mental health relevance of the proposed research flows from the presumed role of temperament and emotionality, and their biological substrates, in the etiology of childhood behavioral disorders. Individual characteristics identified in the study or normal development might act as risk factors for some information to parents of twins, as well as to health car professionals.